Multi city travel

Our son’s family did the math and found that the Friends and Family Railcard worked well for their UK train journeys. It costs £30 but the savings were more. https://www.familyandfriends-railcard.co.uk/?_ga=2.24524734.543442263.1677085398-967122448.1677085397

If you drill down through the terms and conditions you’ll read that the 15yo turning 16 will still be eligible as long as the card is within its one year usage. Do the math on the https://www.nationalrail.co.uk/# website to see if this is a good fit for your trip.

You’ll have to book each train journey separately. There isn’t something like a travel card for national rail. The Friends and Family card gives 30% off most fares except the very cheapest advance fares which you are probably too late to get now anyway.

I've just been looking. Advance fares are available for the Stratford to Manchester, Manchester to York and York to Edinburgh legs, but are becoming limited as that is Easter Weekend.
So I would suggest that you need to get those booked pretty soon.
Chiltern Railways have not yet opened advance fares for the London Marylebone to Stratford leg, so you will only see fares of £34.80 for each adult.
When they do you should get about £10.
That is the least important leg, so I would book the others now, then set up a ticket alert on Chiltern Railways website, ready to pounce when released.

A maths question.

What about combining some rovers with one-way ticket to cover the gap? There are some discounts on railcard holders, incl. family and friends railcard.

One way, preferably advanced ticket from London Marylebone to Leamington Spa.

West Midlands day ranger- between Leamington Spa and Straford-upon-Avon ; between Stratford-upon-Avon and Crewe
http://www.railrover.org/pages/west-midlands-day-ranger.html

A one-way ticket between Crewe and the boundary of the Greater Manchester wayfarer, say Wilmslow?

http://www.railrover.org/pages/greater-manchester-wayfarer.html

Let me see if I have this right....

April 7--London to Stratford upon Avon to Manchester

April 8-- Manchester to York

April 9--York to Edinburgh

That's three travel days in a row. What are you planning to see in Manchester and York?

From Crewe to Manchester it's not worth splitting your ticket on Wilmslow as you can widely get a £3.50 advance ticket from Crewe to Manchester and even a walk up full fare is only £9.80

There are Family Tickets available from London to Stratford upon Avon covering you all for £54.80. This is a return ticket but you only will be using the outward portion. Check for Advance Singles later which will probably be cheaper
Suggest you consult www.seat61.com for information on rail travel in UK

I've just been reading a summary of Easter engineering work. It looks as if Advance Tickets on the Chiltern route may not be made available at all as the line is expecting to be very busy due to the closure of the West Coast Main line between London Euston and Milton Keynes for engineering, also engineering work closures of the entire Jubilee Line and a large part of the Metropolitan London Underground lines. Thus a lot of passengers from those lines are expected to divert to the Chiltern,
So it looks like it is not worth waiting for advance tickets for Marylebone to Stratford, and I would suggest booking that journey now, not waiting.